linux-kernel - [PATCH] memcg: fix wrong VM_BUG_ON() in try

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Message-ID: <alpine.LSU.2.00.1012291355080.22872@sister.anvils>
Date:	Wed, 29 Dec 2010 14:07:11 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Thomas Meyer <thomas@...3r.de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: [PATCH] memcg: fix wrong VM_BUG_ON() in try_charge()'s mm->owner
 check

From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

At __mem_cgroup_try_charge(), VM_BUG_ON(!mm->owner) is checked.
But as commented in mem_cgroup_from_task(), mm->owner can be NULL
in some racy case. This check of VM_BUG_ON() is bad.

A possible story to hit this is at swapoff()->try_to_unuse(). It passes
mm_struct to mem_cgroup_try_charge_swapin() while mm->owner is NULL. If we
can't get proper mem_cgroup from swap_cgroup information, mm->owner is used
as charge target and we see NULL.

Cc: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Reported-by: Hugh Dickins <hughd@...gle.com>
Reported-by: Thomas Meyer <thomas@...3r.de>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Reviewed-by: Balbir Singh <balbir@...ux.vnet.ibm.com>
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Cc: stable@...nel.org
---
Sorry, I hit this on 2.6.36, and we lined up this patch early in
November, but never really pushed it: now Thomas hit it on 37-rc7.

 mm/memcontrol.c |   19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

--- 2.6.37-rc8/mm/memcontrol.c	2010-11-29 22:29:32.000000000 -0800
+++ linux/mm/memcontrol.c	2010-12-28 21:42:29.000000000 -0800
@@ -1925,19 +1925,18 @@ again:
 
 		rcu_read_lock();
 		p = rcu_dereference(mm->owner);
-		VM_BUG_ON(!p);
 		/*
-		 * because we don't have task_lock(), "p" can exit while
-		 * we're here. In that case, "mem" can point to root
-		 * cgroup but never be NULL. (and task_struct itself is freed
-		 * by RCU, cgroup itself is RCU safe.) Then, we have small
-		 * risk here to get wrong cgroup. But such kind of mis-account
-		 * by race always happens because we don't have cgroup_mutex().
-		 * It's overkill and we allow that small race, here.
+		 * Because we don't have task_lock(), "p" can exit.
+		 * In that case, "mem" can point to root or p can be NULL with
+		 * race with swapoff. Then, we have small risk of mis-accouning.
+		 * But such kind of mis-account by race always happens because
+		 * we don't have cgroup_mutex(). It's overkill and we allo that
+		 * small race, here.
+		 * (*) swapoff at el will charge against mm-struct not against
+		 * task-struct. So, mm->owner can be NULL.
 		 */
 		mem = mem_cgroup_from_task(p);
-		VM_BUG_ON(!mem);
-		if (mem_cgroup_is_root(mem)) {
+		if (!mem || mem_cgroup_is_root(mem)) {
 			rcu_read_unlock();
 			goto done;
 		}
--
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